When I use an mp4 file with h.264 video, I cannot control the sound. But when I use an FLV video with the exact same code, it works just fine.

FYI, I also tested with Xcode Instruments to confirm this. When using the FLV, Instruments shows high CPU load, but when using mp4 the CPU load is very load. Yes, that's the whole point of stagevideo, but unfortunately can't control audio.